Adjustment

The synchronizing chain (also known as the "timing" chain) on a tandem is usually adjusted by an eccentric-mounted bottom bracket at one end or the other. When you service a tandem, or, especially, when you set up a new one, you may notice that the tension on the synchronizing chain varies as the cranks turn. It may be quite tight when the cranks are in one position, and rather droopy after the cranks have rotated a bit. This is not good.

The problem is that the chainwheels are not perfectly concentric with the bottom brackets. It is usually possible, and not very difficult, to correct this.

It is best to disconnect the primary chain for this adjustment. This will make it easier to judge how freely the cranks turn. (When the synch chain is too tight, it has a noticeable tendency to bind up the drive train.)

Set the eccentric so that the chain pulls taut at the tightest part of the cranks' rotation. One at a time, loosen up each of the stack bolts on each of the synch chainwheels, and tighten it back just finger tight. Spin the cranks slowly and watch for the chain to get to its tightest point. Strike the taut chain lightly with a convenient tool to make the chain rings move a bit on their spiders. The rotate the cranks some more, finding the new tightest spot, and repeat as necessary.

This takes a little bit of practice, until your hands learn how hard to hit the chain, and how loose to set the stack bolts, but it is really quite easy to learn.

Tighten up the stack bolts a bit and re-check. Tighten the stack bolts in a regular pattern, like the lug nuts on a car wheel. My standard pattern is to start by tightening the bolt opposite the crank, then move clockwise 2 bolts (144 degrees), tighten that one, clockwise 2 more, and so on. Never tighten two neighboring bolts in a row. You may prefer to go counterclockwise, but try to get in the habit of always starting at the same place and always going the same way. This reduces the chances of accidentally missing a bolt.

This technique is also good for high-end BMX bikes and fixed gears, as long as the crankset has replaceable chainrings.

Once you have the chainrings centered and secured, adjust the eccentric to make the chain as nearly tight as possible without binding. Notice how freely the cranks turn when the chain is too loose. That is how freely it should turn when you are done, but with as little chain droop as possible.

Types of Eccentrics

There are three common mechanisms for adjusting eccentrics, each with advantages and disadvantages. All three types work well, with reasonable care, and none of the designs would be a reason to reject one model of tandem or another.

Pinch-bolt eccentrics have splits in the outer bottom bracket shell, with a pair of binder bolts that squeeze the frame shell tight, in the same manner as a seatpostbinder

This is the easiest type to adjust, but is normally only used on steel frames because of the risk of breaking off the ears that the bolts go through. It is also heavier and less attractive.

Set-screw eccentrics have threaded holes in the bottom bracket shell, with set screws that can press against the eccentric, holding it in place.

This is the easiest type to manufacture, and is quite reliable. This system does have a couple of drawbacks: The ends of the set screws can dent the outer surface of the eccentric, making very fine adjustment a bit difficult. Since it relies on threads cut into the frame, if these threads get damaged, you could be in big trouble. This is rarely a problem in practice.

Wedge-type eccentrics fit frames with a plain cylindrical bottom bracket shell. The eccentric is sliced at an angle at one end, and a wedge, similar to that of a handlebar stem wedge bolt, presses against the inside of the shell. This is a good system, easy to adjust if properly maintained and lubricated. If left too long without service, the wedge can become corroded in place, and may be difficult to dislodge without damage.

To loosen a wedge-type eccentric, remove the bolt (usually a 6 mm thread) that holds the wedge in place, and temporarily replace it with a considerably longer one. Thread the bolt well into the wedge, and rap it with a mallet to drive the wedge out. If you try to do this with the normal, short bolt, there may not be enough threads engaged once you have loosened the bolt, and the threads may be damaged by the impact.

Idler tensioners are used on cheap or homebuilt tandems as an alternative way of regulating the chain tension. They consist of an adjustable pulley which can move up and down to push the middle of the un-loaded part (usually the bottom run) of the synch chain out of a straight line, to take up slack. These are less efficient than eccentric systems, because they add an extra moving part.

Idler tensioners are definitely the low-rent way to go, and, if you find yourself choosing between two tandems, one with an eccentric of any type and the other with an idler tensioner, the balance should weigh heavily in favor of the tandem with the eccentric.

Synch Chainrings

Large or Small?

In theory, it makes no difference what size chainwheels you use for the synch chain, as long as they are both the same. The mechanical advantage is the ratio of the two sizes, and if they are both the same size, the mechanical advantage is always 1:1.

For most tandem applications, the synch chainrings must be the same size, because if they are different, one will turn faster than the other, and the phase will be constantly changing. On some tandems, this can cause the riders' feet to collide when the cranks are out of phase.

If you use small synch chainrings, you can save a bit of weight, since the smaller rings are lighter, and you can use a slightly shorter chain.

This is not usually a good idea, however, because the smaller the chainrings, the harder the synch chain pulls (to make up for the fact that it is not moving as fast.) This causes accelerated wear to the chain, chainwheels, and crossover bottom bracket. On less rigid tandems, the greater chain tension of small diameter sync rings can cause noticeable flex of the keel tube, due to "bow and arrow effect" with a strong pilot.

For two-person tandems, chainrings in the range of 38-42 teeth are normally most appropriate, but for older/cheaper tandems larger synch chains may be more efficient.

Worn Out Chainrings?

Synch chainrings last a long time, due to their large number of teeth and straight chainline, but they do eventually wear out. A useful trick when they do is to remove the chainrings and re-install them, with the former front ring on the back, and the former back ring on the front. Since the front (driving) synch ring only wears on the front of the teeth, and the rear (driven) only wears on the back, this brings brand new, undamaged tooth surfaces into action...make sure to replace the chain when you do this!

Odd or Even?

One can make a case that it is better to use even numbers of teeth for synch chains, and keep the chain always set the same way on them. As a chain wears, it elongates, but the elongation only happens between the rollers separated by outer links.

As the chain wears, it deforms the chainweel teeth to match the increased pitch of the worn chain. On an odd numbered chainweel, each tooth is alternately in contact with a "stretched" half link and a normal-pitch half link, every other revolution. The "stretched" half links deform all of the teeth, which then no longer mesh properly with the un-stretched half links.

With an even numbered chainring, only the teeth that correspond to the "stretched" half links get deformed, and by doing so, they work fine with the elongated half links. The alternate teeth don't wear as much, since they are dealing with normal pitch half links.

As long as you don't derail the chain and put it on out of its normal phase relation to the sprocket teeth, a considerably worn chain/chainrings can run smoothly and efficiently.

Since the eccentric has only a limited range of adjustability, in some cases, it may not be possible to use even-size synch chains: if you replace a pair of 39 tooth rings with a pair of 40s, you need to adjust the eccentric 1/4" closer to the fixed bottom bracket, or you can add a link to the synch chain and move the eccentric 3/4" farther. Some eccentrics may not permit this amount of adjustment.

Phase

The usual setup for a tandem synch chain has the two cyclists pedaling in exact synchronization, to when the captain's right pedal is straight up the stoker's right pedal will also be straight up. This is called "in phase."

If the cranks are at different angles from one another, they are described as "out of phase" sometimes with an angular measurment. For instance, if the captain's right pedal is at the top of the stroke while the stoker's right pedal is at the bottom of the stroke, that would be 180 degrees out of phase. (Nobody uses 180 degrees out of phase, it just makes the bike handle weird with no benefit.)

if the captain's cranks are horizontal when the stoker's cranks are vertical, that would be 90 degrees out of phase.

A minority of tandemists prefer "out of phase" setups, usually 90 degrees, with the captain's cranks 90 degrees forward of the stoker's. The advantage touted for this system is that it smooths out the power curve so that one of the riders is able to exert full force on one of the pedals at all times. Some folks believe this offers benefits in climbing, but I don't know of any evidence to support that theory.

90 degrees out-of-phase setups also reduce maximum stresses on the frame and drivetrain, so parts mighg last a bit longer this way.

There are a couple of serious drawbacks to this system:

The riders must exercise great care in cornering to avoid striking a pedal.

The riders body movements are out-of-synch, which reduces the feeling of "togetherness" and also can complicate steering/handling especially at slow speeds.

Some tandemists prefer a setup where the cranks are only slightly out of phase, typically with the captiain's cranks just a few degrees ahead of the stoker's cranks.

Fortunately, no hardware changes are required if you wish to experiment with out-of-phase cranks. It's just a matter of removing and re-installing the chain.

There are essentially three entities riding a Tandem:
The captain, the stoker, and the spirit.
It is the spirit who likes in-phase cranks.